Depending on whether you're using a POE-aware environment or not, people wanting to tinker with mpd (Music Player Daemon) will use either POE::Component::Client::MPD or Audio::MPD.
But even if the run-cores of those two modules differ completely, they are using the exact same common classes to represent the various mpd states and information.
Therefore, those common classes have been outsourced to Audio::MPD::Common.